Today I was working on a book with a quotation from a nineteenth-century Englishman; the quotation had infallible as an adverb. So I looked infallible up in the OED to see whether it was used as an adverb at the time. It wasn't, but it was obsoletely used as a verb long ago. My favorite exemplary quotation is from The Golden Law (1656), by S. H. (Samuel Harsnett):
We will next pursue it with right Reason which will selfly infallible it.
No big deal. I just liked it and wanted to share it.